The present invention relates generally to optical communications, and more particularly, to transmitter compensation in decoding of intensity modulated optical OFDM with direct detection.
Orthogonal Frequency Division Multiplexing (OFDM) is a promising technology in optical transmission because of its tolerance to chromatic dispersion (CD) and polarization mode dispersion (PMD). Compared to other modulation schemes and coherent detection, intensity modulation (IM) with direct detection (DD) has the advantage of reduced system complexity and cost. Because of the modulator and detector non-linearity, and the sensitivity of OFDM signal to this non-linearity, for intensity modulation with direct detection, IM/DD, it is usually good to limit the modulating signal and the received optical signal in a certain (small) range. Moreover, intensity modulation requires the signal to be DC-biased, which further limits the signal amplitude. This leads to inefficient use of the optical power, which further reduces the signal to noise ratio (SNR). Lowering the DC level will cause signal clipping, which results in worse performance.
One solution to the inefficient use of optical power is to limit the signal within the linear range of the modulator (including the amplifier) and the detector. Another approach is to avoid DC bias by clipping all the negative amplitude to increase the signal range, but this doubles the signal bandwidth.
Accordingly, there is a need for a method which increases the optical power to increase the noise tolerance, with improved system performance (lower bit-error-ratio at the same noise level).